Iron bearing residues are a typical by-product of the non-ferrous industry, particularly of the zinc processing industry. Indeed, a well-known technique of zinc production encompasses the electrolysis of a zinc sulfate solution. One of the main impurities that has to be removed from the solution before electrolysis is iron. To this end, iron is precipitated and separated from the zinc solution, forming an iron bearing residue. This residue contains the major part of the iron initially present in the solution, a significant amount of lead, arsenic, silica, and residual zinc. Depending upon the conditions prevailing before and during the separation, the iron in the residue is obtained as jarosite, goethite, hematite or magnetite. Particularly jarosite and goethite have no commercial value and are considered as hazardous waste. The waste disposal sites are to be severely controlled and protected against impregnation by the leachate.
Stabilization and solidification of hazardous industrial waste is a widespread environmental technique, generally described in "Stabilizing hazardous waste", J. R. Conner, Chemtech, December 1993, pp. 35-44. Most inorganic stabilization and solidification techniques use pozzolanic reactions, i.e. reactions of the type occurring in Portland Cement, forming complex hydrated systems between CaO, Al.sub.2 O.sub.3, SiO.sub.2, MgO and Fe.sub.2 O.sub.3.
A known application of this technique in the zinc industry is described in EP-A-0031667. This document deals specifically with the treatment of jarosite, and proposes a method of solidifying it by admixture with calcium containing cement powder and a powder based on alumina and silica (fly-ash). The product has a claimed compressive strength of 0.64 MNm.sup.-2 after 28 days of hardening and exhibits low leachability.
This jarosite treatment process has however certain disadvantages:
the obtained product has a relatively low compressive strength making the product suitable for piling but unsuitable for the building industry; PA1 the important issue of the leachability of lead is not mentioned; and PA1 a substantial amount of Portland Cement is added which degrades the economy of the process. PA1 a mixture is prepared consisting essentially of 1 part of the wet residue mixed with at least 0.1 parts by weight of crushed blast furnace slags and with at least 0.1 parts by weight of crushed converter slags; PA1 water is added to the mixture to obtain a stiff paste; and PA1 the paste is allowed to harden, while kept wet, to such an extent that the resulting rock is usable for construction purposes.
The purpose of the present invention is to bring a process for the conversion of iron bearing residues from the non-ferrous industry into a synthetic rock, which avoids the disadvantages of the process disclosed in EP-A-0031667.
To this end, according to the invention: